The present invention relates generally to testing of integrated circuits, and more specifically to a technique for determining whether a probe has made electrical contact to a node in a device under test.
It is well known practice to test integrated circuit devices prior to dicing the wafer and packaging the chips. In a typical test environment, integrated circuit testing typically involves a wafer sort, carried out before dicing, wherein the wafer is placed on a vacuum chuck and each die is tested. Testing a given die entails bringing a probe card with a number of probe tips into registration with the die so that each bond bad is contacted by a corresponding probe tip. Thereafter, various voltage levels, signals, and combinations of signals are applied to the inputs of an integrated circuit device under test ("DUT") and the DUT's response at its outputs is analyzed. Testing at this level only requires access to the DUT's bond pads, which are large. However, due to the large number of probes (say 100), establishing simultaneous contact is a vexing problem.
Moreover, such testing does not always provide the complete picture. It may often become necessary to determine the signal response at one or more internal circuit nodes. Access to internal nodes is sometimes provided by test pads, which are typically much smaller than the bond pads. Under some circumstances, it may even be necessary to probe the conductive paths on the active area of the chip.
A typical approach uses a probe having a fine wire tip. In present circuit technologies, the conductive paths are very narrow, and direct probing requires steps to avoid damage to the chip. More particularly, it is important to avoid excessive mechanical force, while still providing a method of accurately determining when suitable electrical contact has been made.
In one approach, contact may be ascertained by visual inspection. When the probe makes contact, further downward pressure causes it to slide along the surface of the chip, which can be detected visually. This method is not practical in an automated test environment.
One possible approach for automatic contact sensing uses a probe having two closely spaced tips, with a voltage generating and current detecting circuit connected in series between the probes. When both probe tips make contact with the device, the loop is closed, and current is detected.
This approach is presumably viable for some applications, but has a number of limitations. For example, unless the two tips make contact at the same time, the first tip to contact may exert excessive force before the second tip contacts. Further, the contact area is greater (which may be acceptable for probing bond pads). Furthermore, the dual-tipped probe introduces extra capacitive loading, even if the contact sensing circuitry is disconnected by relays when the probe is used in the measurement mode. Moreover, the dual-tipped probe is not suitable for use as an active probe (having a transistor amplifier located close to the actual probe tip), since it is impossible to make contact to the probe on the DUT side of the transistor.
Thus, it can be seen that despite the sophistication of the actual test equipment, the techniques for determining when the probe has actually made contact with the DUT are relatively crude. An Achilles' heel of sorts.